The present invention relates to non-destructively testing nuclear fuel rods and particularly to determining that nuclear fuel rods have been manufactured in strict accordance with engineering specifications.
A nuclear fuel rod is comprised of a column of fuel pellets sealed in an elongated cladding tube formed of a zirconium alloy. The several hundred fuel pellets are typically in the form of sintered ceramic tablets approximately one centimeter in both diameter and length. A typical nuclear fuel for use in water cooled and moderate power reactors comprises uranium dioxide (UO.sub.2) in which from about 0.7 to 5.0 percent by weight is fissile U-235 mixed with fertile U-238. The pellet column is maintained compacted by a spring acting in a tube plenum chamber which also contains a suitable getter. The fuel rods are arranged in groups and supported as separately replaceable fuel assemblies in the reactor core. A typical fuel assembly is shown in U.S. Pat. NO. 3, 689,358.
To decrease local power peaking and to improve power distribution throughout the fuel assembly, it is common practice to vary the U-235 pellet enrichment concentration along the length of the rod. This is typically done with respect to groups or zones of pellets of various lengths, and a fuel rod may have as many as seven such pellets zones of varying enrichment concentrations within the range of 0.7 to 5 percent. It is also common practice to include in a fuel assembly a number of fuel rods containing a strong neutron absorbing material or "burnable poison", such as gadolinium. By appropriately locating predetermined concentrations of this burnable poison within a fuel assembly, enhanced control of the nuclear reaction is afforded, and thus less reactor control by the mechanical positionings of neutron absorbing control rods is required. Reactor shutdown safety margin is also improved. Moreover, the power distribution profile of a fuel assembly can be more efficiently optimized and maintained over a longer operating life. Typically, a neutron absorbing poison, such as gadolinium oxide (gadolinia) is mixed in with uranium oxide (urania) in concentrations up to 5 percent by weight prior to sintering into pellet form. As with enrichment, gadolinia concentration varies from pellet zone to pellet zone within a fuel rod.
A fuel assembly is thus made up of a number of pure urania fuel rods of designed varying enrichment concentrations from rod to rod and from zone to zone within each rod and a lesser number of poisoned or urania-gadolinia rods also of varying enrichment and gadolinia concentrations from rod to rod and zone to zone. These concentrations and their pellet locations within the fuel assembly must conform to strict engineering specifications in order to assure reactor safety and regulatory compliance, as well as achieve the above-noted reactor performance improvements. It is thus critical that the fuel rods be manufactured strictly to these engineering specifications. Each and every pellet must be in its proper place within each fuel rod. Rigid quality assurance is thus imperative, requiring precise non-destruction testing of each and every fabricated fuel rod. This testing must include, among other properties, accurate determinations of enrichment and poison concentrations at a multiplicity of points along the length of the rod, fuel density, uranium weight in each zone, zone length, out-of-specification pellets, and the presence of the spring and getter.
An early approach to non-destructively determining the enrichment concentration of a non-poisoned fuel rod involved a "passive scanning" method, wherein gamma rays of characteristic energy emitted during the natural radioactive decay of fissile material such as U-235 was quantitatively detected. Since U-235 has a relatively long half-life, its spontaneous disintegration rate and the consequent gamma ray emission rate is low. Consequently, the scanning time required for accurate enrichment quantitative analysis was extremely long, even when the rods were scanned by a distributed array of gamma ray detectors and their counts accumulated. Since every fuel rod must be tested, manufacturing throughput is severely reduced by this passive scanning approach.
To dramatically reduce the testing times for non-poisoned fuel rods, an "active scanning" approach has been adopted. This mentod involves moving a fuel rod past a source of neutrons of selected energy, and then past a detector quantatively responsive to the gamma radiation resulting from the neutron-induced fission of U-235. Knowing the radiation count at each location along the length of the fuel rod, the enrichment concentration thereat can be readily calculated.
Non-destructive testing of poisoned fuel rods presents a considerably more formidable task and thus have been analyzed separately from non-poisoned fuel rods by different quality assurance systems. The presence of a poison, such as gadolinia, has heretofore been deemed to render active scanning ineffective, since the poison acts as a strong neutron absorber and thus absorbs the major portion of the activation neutrons which would otherwise induce fission of the U-235 fissile material. Thus, active scanning systems have heretofore not been able to distinguish whether a detected gamma ray count originated from a fuel rod containing high concentrations of both enrichment and poison or one containing low concentrations of both enrichment and poison. This confusion is further complicated by the fact that enrichment and poison concentrations typically vary from zone to zone along the length of the fuel rods.
Consequently, commercial non-destructive testing of poisoned fuel rods has been limited to quality assurance systems using the passive scanning approach, such as described and claimed in commonly assigned Arya et al. U.S. Pat. No. 4,229,654, the disclosure of which is specifically incorporated herein by reference. As disclosed therein, a fuel rod containing gadolinia-poisoned urania pellets is scanned by passing it through a magnetometer acting to determine from detected changes in paramagnetic susceptibility upon movement in a strong magnetic field the concentrations and distribution of the poison, typically gadolinia. The fuel rod passes a ultrasonic sensor providing tube wall thickness data and a densitometer providing fuel density data, on corresponding data point by data point basis. The fuel rod then passes a distributed array of up to eight detectors responsive to the gamma rays emitted from the fissile, U-235 content of the fuel due to natural radioactive decay. The gamma ray counts of the plural detectors are combined for hundreds of distinct data points distributed along the rod length. The accumulated counts of all the detectors are analyzed and corrected for poison concentration, density, tube wall thickness, and other factors to determine the enrichment concentration at each data point location along the fuel rod length. While this passive scanning system is extremely accurate, it is also slow, requiring a scanning time of approximately ten minutes per gadolinia rod as compared to a scanning time of less than three minutes per rod using an active scanning system. Since the current trend is to use more and more gadolinia fuel rods in fuel assemblies, the slow passive scanning time to non-destructively test them is increasingly becoming a severe production bottleneck. In addition, it is extremely expensive to design, build, operate and maintain one system to test gadolinia-bearing fuel rods and another separate system to test pure urania non-poisoned fuel rods.
It is accordingly an object of the present invention to provide a single quality assurance inspection system and method applicable to non-destructively testing both poisoned and non-poisoned nuclear fuel rods.
A further object is to provide a quality assurance inspection system and method of the above-character, wherein the time required to non-destructively test a poisoned nuclear fuel rod is materially reduced.
An additional object is to provide a quality assurance inspection system and method of the above-character, wherein the time required to non-destructively test a poisoned nuclear fuel rod is reduced to substantially equal the time required to non-destructively test a non-poisoned nuclear fuel rod.
Another object of the present invention is to provide a quality assurance inspection system and method of the above-character, wherein both poisoned and non-poisoned nuclear rods are non-destructively tested using the same scanning approach.
A still further object is to provide a quality assurance inspection system and method of the above-character wherein both poisoned and non-poisoned nuclear rods are actively scanned pursuant to determining the U-235 enrichment concentrations along the lengths of their fuel columns.
An additional object is to provide a quality assurance inspection system and method of the above-character wherein both poisoned and non-poisoned nuclear fuel rods are actively scanned pursuant to determining the grams of uranium and grams of U-235 in each of the fuel zones of the fuel columns.
Yet another object is to provide a quality assurance inspection system and method of the above-character which is implemented by a single non-destuctive testing system concurrently accepting both poisoned and non-poisoned nuclear fuel rods for separate analysis.
Other objects of the invention will in part be obvious and in part appear hereinafter.